In the plating of dielectric substrates by chemical (electroless) plating it is well known that suitable catalytic pretreatment is a prerequisite for effective electroless metal deposition. Such practices are well known and accepted in the art.
In examining the prior art for catalytic pretreatment it appears that while different procedures have been used, the incorporation of precious metals (e.g., palladium containing solutions) was common to all procedures. One catalytic system of particular interest is the two step process as disclosed in U.S. Pat. No. 3,011,920. In the process disclosed, a colloidal solution composed of tin(II) and precious metal salts, generally with hydrochloric acid, is used. The effective catalyst is proposed to be a colloid of an elemental precious metal (e.g., palladium) stabilized by the excess stannous chloride present in the medium. While the system disclosed in U.S. Pat. No. 3,011,920 has been quite popular in commercial practices, rising costs of precious metal, instabilities due to air oxidation, and miscellaneous product reliability problems have led to the quest for new systems in which the use of precious metal as well as hydrochloric acid would be completely eliminated.
In meeting this objective it was found, as described in U.S. Pat. Nos. 3,958,048, 3,993,491, 3,993,799, and 4,087,586 that colloidal systems based upon non-precious metals could constitute the basis for new commercial plating processes. More specifically, it was found and reported that colloidal compositions of non-precious metals (preferably selected from the group of copper, cobalt, iron, nickel and manganese) may be used in the direct replacement of the tin/palladium colloid followed by a treatment (which may be optional) in a suitable reducing medium (activating medium). In the latter treatment a precursor derived from the colloidal dispersion constitutes the catalytic sites useful in the initiation of plating. In the reducing medium, reduction of the ionic portion of the adduct derived from adsorption in the colloidal medium takes place, or surface activation, which results in active nucleating sites capable of initiation of the electroless process. Alternatively, the second step may encompass the selective dissolution of a colloid stabilizer(s) thereby exposing the catalytic nucleus of the colloid, or contacting with a solution comprising soluble compound(s) of catalytic metal. Hence, it should be recognized that the step of activation which is optional generally is intended to reduce the induction time for the actual electroless metal build-up or to remove weakly adsorbed colloids, thereby preventing their contaminating the plating bath(s).
The colloidal nucleus may be in a form of a metallic (elemental) state, or compound bearing the metal, or alloy, and mixtures thereof and the metal(s) must be of a catalytic nature in at least one of its possible oxidation states.
In reviewing the teaching disclosed in the aforementioned issued patents which are included herein by reference, it is recognized that many of the inherent disadvantages associated with the palladium based catalysts are eliminated. It is further recognized that based upon practices in this art it is further essential that any catalytic system should maintain its properties especially with storage (e.g., several months) and shipment under conditions of substantial temperature fluctuation. It is thus highly desirable to have a medium in which good colloidal stability would be maintained, and which at the same time has sufficient catalytic activity to be used in the plating process. I have observed that as one increases stability, activity is decreased thereby making it difficult to meet both requirements in a single preparation step.
For example, I have observed that with successful synthesis of active plating colloids, there is generally a limited stability (for long term purposes) due to coagulation which takes place leading to precipitation, with, of course, change in particle size and distribution during the coagulation process. Also, at times dissolution of the colloidal state may also take place with time. In addition, I have noted that highly stable colloidal dispersions have shown limited catalytic activity when used in accordance with U.S. Pat. No. 3,993,799 with a moderate concentration of reducing medium or activating medium or the omission of any secondary step. Similar trends were also noted in U.S. Pat. No. 3,948,048 on the interrelationship between reactivity and stability. In fact, in U.S. Pat. No. 3,958,048 most of the illustrated examples, when repeated, lost their colloidal character and became true solutions within 24 hours probably due to the interaction of the colloid with air or more particularly with oxygen. Many times the deterioration of the colloid is manifested in visible color change(s). Hence, simple visual observation could determine whether a colloid is deteriorating.
It is thus highly desirable to provide stable colloidal dispersions which would maintain their integrity and resist deterioration by precipitation and/or contact with air. Such colloids may be useful in electroless plating process, catalysis, or any other processes utilizing colloids. It is further desirable to obtain dispersions with very fine particle size distributions. Small size dispersions are particularly useful in adsorption processes and catalysis.
While not wishing to be repetitious, the following are included herein by reference: U.S. Pat. Nos. 3,011,920, 3,993,799, 3,524,754, 3,958,048, 3,993,491, 3,993,801, 4,087,586, 4,048,354, British Pat. No. 1,078,439, and copending applications Ser. No. 712,131 now U.S. Pat. Nos. 4,136,216, 820,904 now U.S. Pat. Nos. 4,131,699, 833,905 now U.S. Pat. Nos. 4,151,311 and 854,909 now U.S. Pat. No. 4,132,832.